Hot weather stalling/ even when coasting to stop on 1998 Chevrolet Express 3500

Drove 1000mi to FL last week. Now in FL. Engine stalls when driven midday (90, sunny, humid) . Re-starts OK, but stalls at idle, even stalls when coasting to a red light stop. What's happening? All else is normal.

I have this same problem only i have to wait about 5 to 10 min then it will restart and run until i get in heavy traffic again. I have no problems with it during weather 70 degrees or below. .2 dealerships charged me to look at it and it still does it. My starteded similarly to yours. Please help it's driving me crazy....